fix: keep the mail list where it was when coming back to the Pro mail tab

components/email/email-list.tsx

@@ -41,6 +41,7 @@ import { toast } from "@/stores/toast-store";
 import { confirmMailRemoval } from "@/lib/mail-removal-confirm";
 import { markSelectionReadWithUndo, runBatchEmailAction, setSelectionTagWithUndo, toastWithUndo, undoLabels, undoMoveAction } from "@/lib/email-action-toast";
 import { useVirtualizer } from "@tanstack/react-virtual";
+import { measureVisibleRow } from "@/lib/virtual-row-measure";
 import { TagDisplayContext, useMeasuredTagDisplay } from "@/hooks/use-tag-display";
 import { SearchChips } from "@/components/search/search-chips";
 import { isFilterEmpty, DEFAULT_SEARCH_FILTERS } from "@/lib/jmap/search-utils";
@@ -493,8 +494,7 @@ export function EmailList({
     // Keep sub-pixel row heights. The default measurer rounds them, so a
     // row could start up to half a pixel inside the one above it and paint
     // over that row's divider (at 125% or 150% display scaling).
-    measureElement: (element, entry) =>
-      entry?.borderBoxSize?.[0]?.blockSize ?? element.getBoundingClientRect().height,
+    measureElement: measureVisibleRow,
   });
 
 

lib/__tests__/virtual-row-measure.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest';
+import type { Virtualizer } from '@tanstack/react-virtual';
+import { measureVisibleRow } from '@/lib/virtual-row-measure';
+
+const instance = {
+  measurementsCache: [{ size: 120 }, { size: 80 }],
+  indexFromElement: () => 1,
+} as unknown as Virtualizer<HTMLDivElement, HTMLElement>;
+
+const entry = (blockSize: number) => ({ borderBoxSize: [{ blockSize, inlineSize: 300 }] }) as unknown as ResizeObserverEntry;
+const row = document.createElement('div');
+
+describe('measureVisibleRow', () => {
+  it('takes the measured height, sub-pixels included', () => {
+    expect(measureVisibleRow(row, entry(96.5), instance)).toBe(96.5);
+  });
+
+  it('keeps the known height of a row in a hidden list', () => {
+    expect(measureVisibleRow(row, entry(0), instance)).toBe(80);
+  });
+
+  it('reports 0 for a hidden row it knows nothing about', () => {
+    const empty = { measurementsCache: [], indexFromElement: () => 0 } as unknown as Virtualizer<HTMLDivElement, HTMLElement>;
+    expect(measureVisibleRow(row, entry(0), empty)).toBe(0);
+  });
+});

lib/virtual-row-measure.ts

@@ -0,0 +1,18 @@
+import type { Virtualizer } from '@tanstack/react-virtual';
+
+/**
+ * A virtualizer's measureElement with sub-pixel row heights that ignores a row
+ * measuring 0 because its list is hidden. A Pro tab in the background is
+ * display:none and its rows report no height: taken at face value the list
+ * collapses, and coming back it grows row by row and scrolls away from where
+ * it was. No visible row is 0 high, so the known size stands then.
+ */
+export function measureVisibleRow<TItem extends Element>(
+  element: TItem,
+  entry: ResizeObserverEntry | undefined,
+  instance: Virtualizer<HTMLDivElement, TItem>,
+): number {
+  const size = entry?.borderBoxSize?.[0]?.blockSize ?? element.getBoundingClientRect().height;
+  if (size > 0) return size;
+  return instance.measurementsCache[instance.indexFromElement(element)]?.size ?? size;
+}